PTSD-SUD is particularly common following combat exposure, affecting a rapidly increasing number of U.S. military Veterans. The co-occurrence of these disorders presents added challenges to the VA treatment delivery system, presently in need of effective integrated treatments. Veterans with PTSD-SUD experience more severe symptomatology, increased risk of suicidality, poorer quality of life, and poorer response to existing treatments than Veterans with either disorder alone. Furthermore, PTSD-SUD prevents Veterans from reintegrating into society and is associated with occupational and social dysfunction. These findings underscore the need to effectively and efficiently address comorbidity and the complex array of problems with which Veterans present to treatment. One approach is to develop interventions that target mechanisms thought to underlie multiple highly prevalent disorders, such as guilt related to traumatic experiences. Combat Veterans often report experiencing moral injury defined as perpetrating, failing to prevent, or witnessing acts that violate the values they live by in their civilian lives. Veterans who negatively appraise their actions or inaction during combat may experience guilt, a common posttraumatic reaction. Moral injury suggests the inability to contextualize or justify actions and the unsuccessful accommodation of those morally challenging experiences into pre-existing moral schemas, resulting in guilt and shame. Posttraumatic guilt has been implicated as a risk factor for the development and maintenance of several forms of psychopathology including PTSD, SUD, depression, and suicidality. However, to date, treatments for posttraumatic psychological health issues have been primarily disorder specific, with a focus largely on symptom reduction. Therefore, greater understanding of modifiable factors that influence functional impairment and PTSD-SUD is needed to enhance treatment efforts. Mindful Self Compassion (MSC) combines the skills of mindfulness and self-compassion, providing self-soothing skills to respond to difficult thoughts and feelings (including guilt) via meditation. Self-compassion (SC) emphasizes kindness towards one's self, a feeling of connectedness with others, and mindful awareness of distressing experiences. Furthermore, because SC is negatively associated with self-criticism, rumination, thought suppression, anxiety, and depression, and positively associated with healthy psychological functioning, it is well suited to addressing posttraumatic psychopathology, shame, and guilt. This proposal will begin to address a gap in the field's knowledge about MSC, and its role in the treatment of co-occurring disorders in Veterans with moral injury. We will evaluate changes in self-compassion, post-traumatic guilt, shame, PTSD and substance use symptom severity. In addition to symptom reduction, we will focus on functional outcomes (e.g., quality of life, suicidality). Participants will complete assessments at baseline, post-treatment, and 1-month follow-up. This project will allow us to 1) determine the feasibility of recruitment, 2) determine the acceptability of MSC, 3) provide preliminary evidence of the effects of MSC, and 4) refine study procedures and make adaptations to MSC based upon experience gained in the pilot in preparation for a fully powered RCT to test the effectiveness of MSC.